Liquid crystal material currently is used in a wide variety of devices, including, for example, optical devices such as visual displays. A property of liquid crystals enabling use in visual displays is the ability to scatter and/or to absorb light when the liquid crystals are in a random alignment and the ability to transmit light when the liquid crystals are in a ordered alignment.
Frequently a visual display using liquid crystals displays dark characters on a gray or relatively light background. In various circumstances it would be desirable, though, using liquid crystal material to be able to display with facility relatively bright colored characters or other information, etc. on a relatively dark background. It would be desirable as well to improve the effective contrast between the character displayed and the background of the display itself.
An example of electrically responsive liquid crystal material and use thereof is found in U.S. Pat. No. 3,322,485. Certain types of liquid crystal material are responsive to temperature, changing the optical characteristics, such as the random or ordered alignment of the liquid crystal material, in response to temperature of the liquid crystal material.
Currently there are three categories of liquid crystal materials, namely cholesteric, nematic and smectic. The present invention preferably uses nematic liquid crystal material or a combination of nematic and some cholesteric type. More specifically, the liquid crystal material preferably is operationally nematic, i.e. it acts as nematic material and not as the other types. Operationally nematic means that in the absence of external fields structural distortion of the liquid crystal is dominated by the orientation of the liquid crystal at its boundaries rather than bulk effects, such as very strong twists as in cholesteric material, or layering as in smectic material. Thus, for example, chiral ingredients which induce a tendency to twist but cannot overcome the effects of boundary alignment still would be operationally nematic. Such material should have a positive dielectric anisotropy. Although various characteristics of the various liquid crystal materials are described in the prior art, one known characteristic is that of reversibility. Particularly, nematic liquid crystal material is known to be reversible, but cholesteric material ordinarily is not reversible.
It is also known to add pleochroic dyes to the liquid crystal material, for example, for increasing absorption characteristics. However, in the nematic form a pleochroic device has relatively low contrast. In the past cholesteric material would be added to the nematic material together with the dye to improve contrast ratio. See for example the White et al article in Journal of Applied Physics, Vol. 45, No. 11, November 1974, at pages 4718-4723. However, although nematic material is reversible, depending on whether or not an electric field is applied across the same, cholesteric material ordinarily would not tend to its original zero field form when the electric field would be removed. Another disadvantage to use of pleochroic dye in solution with liquid crystal material is that the absorption of the dye is not zero in the field-on condition; rather, absorption in the field-on condition follows an ordering parameter, which relates to or is a function of the relative alignment of the dyes. Moreover, pleochroic dyes are relatively expensive and must be used carefully, usually having to be mixed directly in the liquid crystal material itself.
Usually liquid crystal material is anisotropic both optically (birefringence) and, for example in the case of nematic material, electrically. The optical anisotropy is manifest by the scattering and/or absorption (especially when pleochroic dye is in solution with the liquid crystal material) of light when the liquid crystal material is in random alignment, and the transmission of light through the liquid crystal material when it is in ordered alignment. The electrical anisotropy may be a relationship between the dielectric constant or dielectric coefficient with respect to the alignment of the liquid crystal material.
In the past, devices using liquid crystals, such as visual display devices, have been relatively small. Use of encapsulated liquid crystals disclosed in applicant's above mentioned co-pending applications has enabled the satisfactory use of liquid crystals in relatively large size displays, such as billboards, etc., as is disclosed in such application; and another large (or small) scale use may be as an optical shutter to control passage of light from one area into another, say at a window or window-like area of a building. The present invention relates to improvements in such encapsulated liquid crystals and to the utilization of the light scattering characteristic of the liquid crystal material and reflection, e.g. by total internal reflection and/or optical constructive interference, of the scattered light to increase optical path length in a material containing a relatively small quantity of preferably standard or other non-pleochroic dye. The increased path length through the dye assures the desired coloring of output light. The invention also relates to the use of such material and characteristics, for example, to obtain a relatively bright colored character or information displayed on a relatively dark or colored background in both small and large displays, optical shutters, and so on. Such large displays, shutters, etc. may be about one square foot surface area or even larger. In accordance with the present invention the liquid crystal material most preferably is of the encapsulated type.
As used herein with respect to the present invention, encapsulated liquid crystal material means liquid crystal material in a substantially closed containment medium, such as discrete capsules or cells, and preferably may be in the form of an emulsion of the liquid crystal material and the containment medium. Such emulsion should be a stable one. Various methods for making and using encapsulated liquid crystal material and apparatus associated therewith are disclosed below and in applicant's copending applications, which are incorporated by reference.
To facilitate comprehension of the invention relative to conventional prior art liquid crystal displays, one typical prior art display is described here. Such a prior display may include a support medium and liquid crystal material supported thereby. The display is relatively flat and is viewed from a viewing side or direction from which a so-called front or top surface of the display is viewed. The back or bottom surface of the support medium may have a light reflective coating tending to make the same appear relatively bright incomparison to relatively dark characters formed at areas where there is liquid crystal material. (Back, front, top, bottom, etc. are used herein in general and with reference to the drawings only for convenience; there is no constraint that in operation the viewing direction must be, for example, from only the top, etc.). When the liquid crystal material is in ordered alignment, for example in reponse to application of an electric field thereto, incident light from the viewing direction passes through the liquid crystal material to the light reflective coating and also where there is no liquid crystal material passes directly to the light reflective coating; and no character is observed from the viewing direction. However, when the liquid crystal material is in random alignment, it will absorb some and scatter some incident light thereby to form a relatively dark character on a relatively light color background, for example of gray or other color depending on the type of light reflective coating mentioned above, which still continues to reflect incident light where there is no liquid crystal material or where some liquid crystal material is in ordered alignment. In this type of display it is undesirable for the liquid crystal material to scatter light because some of that scattered light will be directed back in the viewing direction thereby reducing the darkness or contrast of the character relative to the background of the display. Pleochroic dye often is added to the liquid crystal material to increase absorbence and, thus, contrast when the liquid crystal material is in random alignment.